Wastewater is generated from common sources, such as homes, schools, restaurants, hotels, office buildings, and the like. From these sources, wastewater enters a collection system and is gravity fed downstream through underground sewer pipes to a municipal treatment plant where the wastewater is chemically and biologically treated for return to the environment. Collection basins containing one or more wastewater pumps are located in areas of low elevation between the sources of wastewater and the treatment plant.
A pump station is generally constructed of concrete or fiberglass, and is a well or container that holds the wastewater liquid, and is typically between four to twelve feet in diameter, and can range from four to over forty feet deep. The pump station receives the flow of wastewater from the gravity sewer pipes that feed it. The pump station also houses one or more discharge pumps that serve to “push” the wastewater to another high point, or directly to the treatment plant. Depending on the plant location, multiple pump stations may be required to transport the wastewater to its final destination for treatment.
Each pump station has some means of signaling when the discharge pumps should turn “on” and “off” depending on the level of wastewater in the basin. Most commonly, this signaling means comprises various float switches staggered at different elevations in the basin, well, or container. These floats are wired back to a central control panel that houses the motor starters for the pumps. Depending on which of the floats “tips” determines which of the pump motors energizes, when it “de-energizes”, and when there is a “high water” or “low water” condition.
Wastewater entering these pump stations typically conveys many contaminants along with the water—in particular, fats, oils and grease (FOG). As wastewater stored in the pump station well or container becomes stagnant, most of this FOG or FOG contaminants rises to the surface and solidifies. This FOG tends to collect on, and build-up around, anything it touches. It is particularly common for grease to collect on the aforementioned floats that control the operation of the pumps. When grease collects on a float, it can weigh the float down or hold it in one position. By not allowing the float to “tip,” operation of the associated electrical switch is restricted, which in turn, affects automatic operation of the pump. Also, grease may “bridge” from a float to a pipe or other structure in the pump station, and the float may “stick” in the “up” position. When this happens, the pump motor never gets its signal to turn off, and continues to run even after there is no wastewater remaining in the basin, well, or container. In this event, the motor overheats causing substantial and costly mechanical damage to the pump.
Other prior art methods of wastewater level measurement include the measurement of electrical conductivity, as water is a conductive liquid. One prior art method of conductivity utilizes a rod, often formed from a non-conductive base material such as plastic, having a plurality of metal contacts (e.g. electrodes) on the rod. This rod is submerged in the pump station wastewater container or well, often near the bottom of the container. Each rod electrical contact (electrode) is connected to a unit in a control panel or other level computing device (which in some embodiments may be a microprocessor and software controlled wastewater level computing device). The control panel or other wastewater level computing device often applies a low AC voltage to each contact or electrode, and checks each electrode for a current to ground (or to another separate reference electrode, which again may be a ground). Here the criterion is that this current must be above a certain user-defined threshold in order for the system to register that the water is at that particular electrode level. Thus, the wastewater level (e.g. liquid level) in the well can be determined depending on the number of and position contacts allowing current to flow to ground.
As can be appreciated, when FOG and other waste material builds up on the rod, the waste materials can form an imperfect electrical seal and or water bridge that can cover and bridge one or more of the metal contacts (electrodes). The FOG waste material bridge or FOG contaminant can throw off the electrode readings. For example, the FOG material can act as a water bridge that serves to retain liquid, and thus enables a false electrical connection between the metal contact and the reference electrode ground to occur even when the water level is low. Alternatively, after the water level has dropped and the bridge has somewhat dried out, capillary action by the FOG contaminant bridge may wick liquid up from rising wastewater up through the waste material. In either of these cases, once the liquid in the well has risen up to the bottom of the waste material bridge, an electrical path capable of conducting the electrical measurement current will be created between ground and all of the metal contacts or electrodes that the FOG waste material has formed over. So the electronic unit may detect a false level—for example it may generate a reading that is higher than the actual wastewater level.
Therefore, when a lower electrical contact electrode on a FOG contaminant covered rod contacts liquid, the FOG bridge also provides an electrical path to a higher contact. This condition produces a false reading and “short cycling” of the pumps in that a wastewater pump will begin operation sooner than it normally would. The result is that the wastewater pumps cycle on and off more frequently than needed, leading to both short-term and long-term pump problems.
A similar problem occurs with the conductivity device as FOG builds up on the rod, especially around the contact or electrode which corresponds to the pump starting. This is because this level is the usually highest wastewater liquid level that is usually reached in the pump station. As the FOG or FOG contaminants are non-conductive, it can prevent an electrical current from flowing even when the liquid level reaches the same level as this contact. In this case, the level will keep rising until a higher contact is reached, but the same problem will eventually occur on this higher contact. In this scenario, the pump motor may not activate to transfer the rising wastewater from the pump station further downstream towards the treatment plant. As a result, the pump station may go into an “alarm” state, and can overflow onto the ground around the well, and into a nearby creek or stream.
To prevent these problems, the conductivity based wastewater level measuring devices should be cleaned with some frequency. Unfortunately, with existing devices, there is often no clear indication whether or not cleaning is required. Conversely, sometimes a large FOG buildup has no detrimental on the system operation, and as a result, the operators often may end up cleaning the rod when it is not required.
Another shortfall of prior art conductivity devices, and of floats, is that the liquid level value is usually measured in steps. For example, for a conductivity based level determination device with ten electrodes, there are at most only ten liquid level “steps” that can be measured.
Here the problem is that it is standard practice to make the liquid level determination rod only a small fraction of the height of the entire well, and to place such liquid level determination rods in the bottom half or bottom third of the well. This is done because this is where the pumps are ideally started and stopped. Unfortunately, once the water level progresses above the highest contact on the comparatively short rod, there is no way of knowing exactly what the wastewater liquid level is rising to in the pump station well or container.
This is a particular problem in high wastewater level conditions. The problem is exacerbated because often sewage treatment plants have multiple such pumping stations, and if the current wastewater management situation has caused one well/pumping station to nearly overflow, in fact often multiple such wells/pumping stations may be about to overflow, or otherwise be in an undesirable high wastewater liquid level condition. To cope with this problem, the operators need to know which station is most likely to overflow first. Here, prior art conducting rod measurement devices usually are not helpful, because they don't tell how high above the highest electrical contact on the rod the wastewater has risen.
Still another problem occurs when prior art, step-electrical contact, measuring rods are used to generate wastewater levels for variable speed drive (known as VSD or VFD) stations. These stations, which are becoming more common, typically work better when the liquid level is known with more detail—e.g. when the level of the wastewater in between the various electrical contacts, and past the range of the highest rod contact, is known. VFDs do not work well with discontinuous “process variables” such as stepped (discontinuous) liquid level readings.